The present invention relates to improvements in power steering cases for steering mechanisms of automotive vehicles, particularly to power steering cases which automatically operate in a completely mechanical mode when the vehicle reaches a predetermined speed, preferably a high speed, with the aim of preventing overassistance of the driver which endangers the maneuverability of the vehicle.
Already well known in practice are the power steering cases for steering mechanisms or servosteering which reduce the effort necessary to turn the steering wheel, especially when the vehicle is stopped, is being parked, or is otherwise being steered at low speeds. These mechanisms work in a manner similar to a conventional gear steering wheel, with the difference that it has a piston solidly connected to the rack bar that is driven, in one or the other direction, by a fluid under pressure selectively supplied through a valve which is integrally formed to the pinion, whose automatic command is carried out by the driver when turning the steering wheel, in each maneuver he makes.
Although these power cases comply with the specific end to which they are directed, especially at low speeds, they also provide help that is detrimental at high speeds where overassistance of the driver occurs. During such overassistance, a sudden maneuver may cause the driver to lose control over the vehicle. This continuous assistance provided by the power steering cases of the steering mechanisms is a source of worry for their manufacturers, who have tried hitherto unsuccessfully, to solve this problem. To this end, functional modifications have been made to these cases, the modifications being limited to controlling the passage of fluid through its distribution valve in order to reduce the assistance effect or to regulate selectively the work of the pressure fluid pump to reduce the supply of fluid; but in all cases the steering mechanisms maintain, to a greater or lesser degree, the assistance to the driver, which means that the above problem subsists at high speeds.
The ideal solution to the problem consists of modifying the power steering mechanisms so that they respond as a power steering case at low speeds, and as an entirely mechanical steering case at high speeds, so that their optimal performance is obtained.
Accordingly, it is an object of the present invention to provide a power steering case for steering mechanisms, which power steering case provides steering assistance until a speed higher than a predetermined value is reached. The power steering case also includes means for progressively and gradually diminishing the angular interplay of the valvular nucleus of the power steering case and of the pinion until power assistance ceases, so as to constitute in this manner an integral assembly between all the components of the power steering case, functioning as a conventional mechanical steering mechanism.
In order to fully interpret the solution proposed, it is advisable to review summarily the structural and functional features of a conventional power steering case, as the one shown in FIG. 12 of the drawings.
A case of this type comprises, in general, a steering casing (1) inside of which a cylinder (2) is incorporated. The cylinder (2) is hermetically closed at one end and a piston (3) is mounted in the cylinder (2). The piston (3) is fixed to the end of a rack bar (4) which is coaxial to the casing (1) and which extends beyond the cylinder (2) to remain operatively related to the vehicle's steering linkage, not shown in the figures, which integrates the steering mechanism with the power steering case.
The casing (1) comprises a case body (1') wherein a valve (5) is mounted for controlling the passage of a fluid under pressure. The valve (5) is constituted by an axle or valvular nucleus (5') rotatably supported inside the case body (1') and by a jacket (5") which is coaxial with the nucleus (5').
The nucleus (5') has orifices (6) in its upper part which communicate with the supply circuit of fluid under pressure and with the interior of the axle or nucleus (5'). The nucleus (5') also has orifices (6') directed toward the jacket (5") . The jacket (5") presents passages (7) for the fluid which communicate, while the power steering case is functioning, with the interior of the nucleus (5'), and which also communicate, by means of conduits, with opposite sides of the piston (3) to provide the power assistance during steering maneuvers.
The axle or nucleus (5') has an upper end portion attached to the column of the steering wheel and a toothed lower end portion (5'a) which meshes with a toothed interior of a bushing (8) mounted in the case (1').
Between the toothed lower end portion (5'a) and the toothed interior of the bushing (8), there is a clearance which allows the nucleus (5') of the valve (5) to have an angular movement at the base of the bushing (8). A pinion (4') projects from the nucleus (5') and is meshed with rack bar (4), the bushing (8) being connected to the jacket (5") of valve 5.
In the interior of the axle or nucleus (5'), coaxial to the same, there is mounted a torque bar (9), affixed at one end to the upper end portion of the axle or nucleus (5'), and at its opposite end to the base of bushing (8) which, in turn, is connected to jacket (5").
The manner of operating this conventional power steering case starts when the steering wheel is turned and the movement is transmitted to the pinion (4') through the torque bar (9). The twisting of the torque bar (9) causes a slight difference in the displacement of the valvular nucleus (5') with respect to the pinion (4'), which difference is reflected in the position of the valvular nucleus (5') relative to the jacket (5") of valve (5), thus permitting the passing of fluid under pressure to one or the other side of the piston (3) of the rack bar (4), thereby obtaining power assistance for the steering effort.